Childhood immunization is one of the most important public health achievements. Unfortunately, the number of infants who do not receive vaccines according to the recommended schedule has grown substantially over the last two decades, leading to numerous outbreaks of vaccine-preventable diseases (VPDs). Causes of infant under-vaccination are multifactorial and include parental attitudes that lead to vaccine hesitancy, a variety of logistical barriers and, mismatch between parents' health values and vaccine recommendations. Interventions to counteract the growing trend of infant under-vaccination are clearly needed. The overarching goal of this application is to develop and test a novel, dynamic, web-based intervention for pregnant and new mothers to reduce infant under-vaccination. Our intervention is based on the concept of message tailoring whereby each mother is provided individually customized vaccine-related information that addresses their unique concerns, questions, values and logistical barriers. Message tailoring is an evidence-based health messaging strategy that has been shown to improve compliance with a wide variety of preventive health care behaviors among diverse patient populations. However, message tailoring approaches not been previously applied to mothers during what research shows to be the most formative stages of maternal vaccination decisions - that is, pregnancy and during the first year of a child's life. To address this gap in knowledge, the following Aims are proposed: 1. To develop a dynamic, web-based, tailored-messaging intervention for pregnant/new mothers to address their vaccination concerns and barriers. The intervention will address multiple types of barriers (i.e. vaccination beliefs, logistical barriers, values) and cn evolve over time to accommodate changes in vaccine availability and controversies. 2. Develop and validate a tool to measure among pregnant/new mothers logistical barriers to vaccination and vaccination-related values. Logistical barriers and maternal vaccination values are important but understudied factors influencing childhood vaccine receipt. This tool will measure the impact of our intervention on these outcomes. 3. To conduct a randomized intervention trial to measure the effectiveness of our intervention. In this three-armed trial we will examine how providing mothers with multiple doses of a tailored versus untailored version of our intervention compares with usual care to impact infant under-vaccination. Secondary outcomes will include changes in maternal vaccine-related knowledge, attitudes and beliefs, alignment of maternal values with vaccine recommendations, and logistical barriers to vaccination.